Minimizing electron optical distortions in electron camera tubes



M. OLIVER 2,535,810

5 Sheets-Sheet l Dec. 26, 1950 B.

MINIMIZING ELECTRON OPTICAL DISTORTIONS IN ELECTRON CAMERA TUBES Filed Aug. 26, 1947 ATTORNEY B. M. OLIVER MINIMIZING ELECTRON OPTICAL DISTORTIONS Dec. 26, 1950 IN ELECTRON CAMERA TUBES 5 sheets-sheet 2 Filed Aug. 26, 1947 BESL Q Q .EEE S Y A T TORNF Y Dec. 26, 1950 B. M. OLIVER MINIMIZING ELEcTRoN OPTICAL DIsToRTIoNs IN ELECTRON CAMERA TUBES 5 Sheets-Sheet 3 Filed Aug. 26, 1947 kk LULU /NVENTOR B. M OLIVER BV 47%; f/T ATTORNEY Dec. 26, 1950 B. M. OLIVER 2,535,810 MINIMzING ELEcTRoN OPTICAL DlsToRTIoNs 1N ELEcTRoN CAMERA TUBES Filed Aug. 26, 1947 5 Sheets-Sheet 4 M. @U1/5R BV Alf s. uw?

ATTORNEY Dec. 26, 1950 B. M. OLIVER 2,535,810

MINIMIZING ELEc'rRoN OPTICAL nIsToRTIoNs `1N ELECTRON CAMERA TUBES Filed Aug. 26, 1947 5 Sheets-Sheet 5 /NVE/VTOR B. M OL/VER BV l )dy/ i d? ATmp/w-v image on the cathode.

Patented Dec. 26, 195() MINIMIZING ELECTRON OPTICAL DISTOR- IION S IN ELECTRON CAlHERA TUBES Bernard M. Oliver, New York, N. Y., assgnor to Bell Telephone Laboratories, Incorporated, New York, N; Y., a corporation of New York Application August 26, 1947, Serial No. 770,639

Claims. l

`This invention relates primarily 4to television transmitting apparatus and more specifically to arrangements including electron camera tubes of the dissector type.

It is an object of this invention to reduce certain distortions heretofore generally produced in television transmitter arrangements employing tubes of the dissector type.

One well-known `form of cathode ray television pick-upY `device is called 'the dissector. In the usual dissector tube, an image of the object is formed on a photoelectric cathode, thereby giving rise to a stream of electrons, various elemental portions of a cross-section of which,

taken at a plane containing a scanning aperture' in front of a pick-up electrode, correspond respectively to the elemental areas of the optical The electrons are accelerated towards the scanning aperture by an axial electrostatic lield. In order to use the tube as a television `pick-up device, three independently variable magnetic fields are usually necessary. First, an axial magnetic eld is required to form an electron image at the end of the tube remote from the photocathode. Next, two transverse magnetic fields, known as the horizontal and vertical deiiecting (or sweep) fields, are required in order to deflect the electron stream from side to side and up and down and thus :displace the electron image in these directions. By means of these two transverse magnetic fields it is possible to move all elemental parts of the electron image in succession over a ilXed aperture located in the image or fscamngplane. The number of electrons received by an electrode .directly behind the aperture depends on the electron density of that portion of the electron image which falls on the aperture, and hence .in turn is proportional to the light intensity falling on the corresponding element of the cathode surface. In certain special applications, such as those in which vtheY :material to be scanned is a moving film, one deilecting field may be omitted. In television systems employing the usual :dissector tube as a pick-up device, it has been noted `that the `picture produced on the screen of 'the receiving tube has been distorted due, at least in part, to the fact that the magnetic sweep coils and, in some cases,

the focussing coil and also the electrode structure producing the electrostatic field in the Idissector have been so formed as to produce non-uniform fields. However, even Vit Aall the fields are made uniform, there are residual distortions in the picture. The present invention, in one of its 'primary aspects, is concerned with the reduction of these latter distortions. One form of such distortion 4is known as S distortion and is easily observed on the screen of a receiving tube used to reproduce an image of the scanned object. This type of distortion gets its name from the fact that the outside of the image is twisted with respect 'to the inside. Another type of distortion is called barrel or pin-cushion distortion and can be easily recognized on the screen of the receiving tube because the image fleld is in the .shape of apin-cushion or barrel. Another type of distortion is a blurring -at the edges and corners of the picture `due to defocussing of the image as it is deflected.

lt has been found that the geometric distortions produced in the dissector type tube can be eliminated or greatly minimized `by using .substantially uniform accelerating, focussing and deflecting elds and by either (l) modulating the electric field by a factor proportional to the sum of the squares `of the two deecting fields, or by (2) modulating both the focussing and `dellecting elds by .a factor proportional to 'the sum of the squares of these two deiiecting iields.

In accordance with an illustrative embodiment of invention employing the first of these two methods of correction, a balanced voltage is derived which is proportional to the vertical sweep field intensity and this voltage is applied to a `vacuum tube device (termed a squarer device) operating on a curved portion of its characteristic in such a manner that its output current proportional to the square of the applied voltage. Similarly, a balanced voltage is developed which is proportional to the horizontal sweep field intensity and this voltage is applied to a second squarer device. The outputs of the two squares are added and amplified to obtain a voltage proportional to the sum of the squares of the two deiiecting elds. Since the deflecting elds are at right vangles to each other, this voltage `is proportional to the quare of the total defiectingeld. Afraction of this voltage is applied to each of `the beam accelerating rings in a ydissector of the type in which the accelerating rings are placed at progressively higher potentials in kthe direction of electron movement, causing their potentials to vary in such a manner that while the electric lield in the dissector vis always substantially uniform throughout the use- 'ful volume, theintensity of this field is increased during deflection of the image by ,an `amount proportional ,to the square of the deflection. It will .be shown mathematically below how .this acts to greatly reduce geometric distortions produced `in the tube.

`In an illustrative embodiment of the invention which makes use of the second method of correction, both focussing and deflecting fields are modulated by a factor proportional to the sum of the squares of the two delecting fields. This is accomplished by using the outputs of the squarer devices already described to amplitudemodulate the sweep currents in both deflection coils and at the same time to modulate the in- Y tensity of the focussing field by varying the curtions broken away, of the dissector tube included in the arrangement of Fig. 1 and its associated sweep and focussing windings;

\ Fig. 3 is a circuit diagram of vertical and horizontal sweep amplifiers, squarer circuits and correction ampliier suitable for inclusion in the .circuit shown in Fig. 1;

Y Fig. 4 is a circuit diagram of vacuum tube voltage divider circuits suitable for inclusionV in the vcircuit of Fig. 1;

Fig. 5 is a diagram of a phase inverter circuit Isuitable for use in the circuit of Fig. 1 when the latter is slightly modified.

Fig. 6is a schematic diagram of a modified circuit for eliminating or greatly reducing distortion in the dissector tube;

Fig. 7- is a circuit diagram of a modulator whic can be used in the circuit of Fig. 6; and f Fig. 8 is a diagrammatic and graphical representation to aid in understanding the invention.

Referring more specifically to the drawings, Fig. 1 shows schematically a television transmitter. station-including a dissector tube I0 while Fig. 2 shows (in enlarged form) the dissector `ltube I2. (with portions broken away) and associated horizontal sweep winding vertical i 3,

sweep winding l2 and focussing winding .which windings. are preferably formed and wound in the manner shown in Fig. V7 of B. M. Oliver Patent 2,278,478, issued April 7, 1942, so that a substantially uniform magnetic lfield is produced within the space enclosed by the dissector i. To assist in the production of a uniform magnetic focussing field, particularly at the ends of the dissector, additional end turns Ill and l5, the purpose of which is well understood by the workers in the art, have been added at the ends of the focussing winding I3. The winds Il, i2 and I3 are supported on suitable forms i3, il and I8, respectively, and the various 'windings are shielded by means of cylindrical electrostatic shields i9, 29 and 2|, respectively.

The dissector tube iii comprises a photoelectric cathode 22 upon which radiations from an object O are focussed by any suitable optical system represented schematically by the single lens 38, an electrostatic accelerating field producing means comprising a plurality of conduct- Iing rings 23 to 3l, inclusive, and a pick-up member 39 at the end of the tube I0 remote from the photoelectric cathode 22. The Vpick-up `member 39 preferably comprises Va metallic finger t having an aperture 4I therein of ele- 'rnental size and an anode 42 back of the aperture for receiving electrons which pass through it. Any suitable electron multiplier (not shown) may be enclosed within the pick-up member 39. In order that the electric field, which accelerates the photoelectrons from the cathode 22 towards the anode t2 in the member 39, be uniform in intensity and axial in direction, the conducting rings 23 to 31, inclusive, are connected respectively to points of different potential on the voltage divider 43 which is shown in detail in Fig. 4. The divider 43, the circuit arrangement of which will be described more fully below, is so arranged that the potentials of the respective rings 23 to 31, inclusive are increasingly larger in the direction of the electron flow. A mesh cap 44 on or near the end wall of the tube l0 is placed at the same potential as the ring 23. The metallic finger i0 is placed at ground potential which is also made the potential of the nearest ring member 25. The anode 42 is connected through resistor 46 to ground. The terminals of the resistor 46 are connected to a transmitter 'il wherein the output signals of the dissector Hl are prepared for radiation, by means of the sending antenna 58; to a receiving station.

Before explaining in detail the manner inV which sweep waves are applied to the horizontal and vertical sweep windings, and alternating components applied to the various potential rings 23 to 3l, inclusive, in the dissector tube l0, the electron optics of the dissector tube will be considered in a mathematical discussion given at this point.

The electron optics of this device is extremely complicated for the general case of inhomogeneous electric and magnetic fields and, so far as is known, has never been completely solved. For the case of uniform fields throughout the volume of the dissector, however, manageable expressions for the electron trajectories can be obtained and a complete solution for the behavior of the device can be worked out. This has been done and the results areY summarized below.V In the following analysis:

zc, y, a are instantaneous coordinates of the electron,

f=force on the electron,

m=mass of the electron,

v=velocity of the electron,

e=charge of the electron,

E=electric field strength,

H=magnetic field strength,

t=time,

q represents any coordinate (a: or y or e) a: a proportionality constant,

subscript T indicates terminal conditions (in imagev piane),

A dot over a symbol indicates the first derivative of that quantity with respect to time,

Two dots over a symbol indicate the second derivative of that quantity with respect to time,

subscripts x, y or .e used with a symbol indicate that the quantity exists in the y or .e direction, and a bar over a quantity indicates the average value of that quantity, or that the quantity is a vector.

cident with Vanyrdesired point on the photocathode Where emission is produced. Also because of the uniformity of the fields, it suffices to consider only one deecting field, since the two deecting elds can everywhere be replaced by their resultant (another uniform transverse field). The coordinate system is assumed to be so rotated that this resultant field lies parallel to the X-aXis. The plane OABC, which makes an angle 6 With the XZ plane, contains the vector representing the initial velocity. In other words the direction of the initial velocity makes an angle 0 with the Z-axis and lies in a plane making an angle e with the XZ plane.

The rst step in the solution consists in finding the timeparametric equations of position of an electron emitted at t=0, x=0, 31:0, and z: 0, With arbitrary initial velocity t: M+ gHa. (ago) The equations of motion of the electron in the fields Ez, HZ and Hx are:

The electron charge, e, in these equations is negative. In the actual case, therefore, EZ Will also be negative.

Making use of the initial conditions that at t=0,

tions for the electron position as a function of time. Thus, letting from which:

The trajectory equations may now be Written:

H. Hat-Hm l ya Hl: H (t-Lw sin wt`)+-5(l-cos stil-f- Now let eH -zw e t2 Elm Thus and the terminal coordinates 'are given by:

Making further use of the relations given above, these equations may be Written:

where satisfies the equation:

These equations may be solved H; T 2 aan) l (-T-o eliminated and the results simplied, making use of the fact that usually d HI 27|' 2 H 27K' y() goil) Wgr H (wat) "LMKH, @To a V2 The rst term in each of the above equations does not contain the initial velocities, and therefore represents the deflection produced on an electron having zero initial velocity. It is also nearly the mean or average deflection of all electrons. About this mean or average deflection there is a dispersion due to the initial velocities given by the second term. The image eld distortion is therefore contained lin the rst terms and the defocussing effects in the second. Accordingly, we may for convenience writer Thus for the mean total deflection, 17T:

7r2=TTZ7T2 and for the radial dispersion, p,

approximately.

. If 17T is not linearly proportional to HX, there will be lbarrel or pin-cushion distortion; if tan is not constant, there will be S-distortion, and

if the factor between brackets in the expression fior p is not constant, there will be defocussing produced by the deection. This is the case if 8 both Hz and E'z are held constant under deection, and all three defects exist.

(a) Correction VVof geometric distortion (first method) If Hz is constant, 5T will be proportional to HX. If ya: were also proportional to Hx, there would be no geometric distortion. For this to be true, H (wTo)2 must be constant (independent of Hx). Y

For H (wTo) 2 to be constant,

must be constant.

3 HJ 3 H.,4 l H 6 a: 3 H Hz 1+2 Hz2+8 Hal-16 HN' If now the electric field were varied as a function of Hx so that H z2 3 er where Vo is a constant, then would be constant and there would be no geometric distortion. Practically, it is sufficient to let with then:

is thus constant to the fourth order which is within the accuracy of the solutions for ET and yr. So long as according to the last equation will be constant to within 0.06 per cent. This is a negligible variation.

(b) Correction of defocussing under deflection (first method) Hz will normally be adjusted so that the term between the brackets in the expression for p vanishes for some average or mean value of o cos 0, say o ccs 0:11a. There will be no defocussing due to deflection if for all Hx,

[cia-e -0 1:1z cuTo L If, as above, the electric eld is-` varied, To and 1v Willzlsvary and these changes can be made to compensate for the changes in H. Since wT=eiI m ZV the above equation may be Written ZIV Vgn- Now let Y H z2 and let v0 be the value of v 'when V=Vn.

Since it followsv that The rst bracket must equal Zero if the picture is to be in focus with no deflection. This requires that Wm Hvo-l-va) The second bracket then becomes Thus for no defocussing under deflection, za-:2, instead of as required for no geometric distortion. However, as may easily be seen, the use of Cab reduces the defocussing under deflection to one fourth of the Value with a=0.

(c) Correction of geometric distortion (second method) It Was shown above that geometric distortion can be substantially eliminated and at the same time defocussing of the image due to deflection can be substantially reduced by suitable modulation of the electric eld in the dissector. Offhand, this would appear the simpler and more straightforward method. However, it is also possible to accomplish the same result by suitably modulating both the deflecting field and the focussing eld, as will be shown. As before In other words both the derlecting and focussing elds must be modulated together.

H Since Ez is constant, To is constant also and yr Will be linear in H1 provided and hence is linear in H1.

and therefore l-l-agg 3mi-H) l2 [HGH 2 )H02 This expression will be linear in H1 if (d) Correction of defocussing under defectz'on (second method) vSince Ez is lconstant ev will be constant also and there will be no defocussing under deflection E EL H z To is constant. Now

f m n fn v in wTode de H H For the image to be in focus with no defiection requires that Since Y 1 1,a;-1 ZV and. neglecting fourth order terms The last term of this last expression is also of fourth order and therefore negligible. Therefore 1t 1s permissible to 'derive the correcting modulation from the (corrected) sweep fields as assumed in the disclosed embodiment of the second method.

u H12 Hz2 (le @c Of the two, method (l) is the simpler. As Vstated at the beginning of this analysis, Hx is considered to be the total deecting field. If Hv is the vertical deflecting field and Hh is the horizontal deflecting field, then, since these two fields are at right angles to each other, HX willat all times be given by Thus to perform the required correction with either of the above two methods, it is necessary to derive a signal proportional to A block diagram of a system employing the first method of correction is shown in Fig. 1 while Figs. 2 to 4, inclusive, are circuit diagrams vof portions of the circuit of Fig. l shown within dashed line rectangles in the latter figure. Vertical and horizontal synchronizing signals of any well-known type used in television systems are formed in generators 59 and 5l, respectively, and applied to Vertical and horizontal sweep generators 52 andv 53, respectively. The devices 52 and 53 can be of any suitable type which produces a saw-toothed voltage wave. Such circuits are Well known in the prior art and no detailed description thereof at this time is necessary. The output saw-toothed waves are applied to vertical and horizontal sweep amplifiers 56 and 55, respectively. Suitable vertical and horizontal sweep amplers are shown in Fig. 3.

The amplifier 54 is a three-stage, low frequency amplifier employing three tubes 55, 57 and 58, respectively. The saw-tooth wave output of the sweep generator 52 is applied to the control grid'of the tube 56. The anode of the tube 58 is connected to the +B terminal throughthe primary winding 59 of a transformer G, the secondary winding 6l yof which is connected to the vertical sweep coil l2 associated with the tube l5. Included in the circuit with the sweep coil and the winding 5| are two equal, seriesconnectedresistors 52 and 63, the common terminal 64 of which is connected to ground. The outer terminals of` the resistors 52 and 53 are connected to the terminals 65 and 56 of the squarer circuit 5l the output of which, aong with the output of squarer circuit S3, is applied to the connection amplifier 69. By using two equal resistors 52 and 63 with their common terminal 64 grounded, a balanced output Voltage wave proportional to the sweep current, and hence to the sweep field intensity, is applied to the squarer circuit 51. "One terminal 'lll of the secondary winding 6I is connected through the connection il and resistor 12 to the cathode of the tube 56, thus providing negative feedback. This feedback tends to make the sweep current wave in the winding l2 faithfully correspond to thesweep voltage wave applied to the input of the tube 5B.

The horizontal sweep amplifier 55 shown in Fig. 3 is a 'high frequency .amplifier and thus employs high frequency compensation. Otherwise it is quite similar to the vertical sweep amplier 54.. The output of the sweep generator 53 is applied to the control grid of the first of the three 'tubes 13, 14 and 15 making up the amplier 55. As a practical matter, the second stage of this amplifier can comprise two tubes in parallel and the third stage can comprise three tubes in parallel. The anodes of the tubes 'i3 and 14 are connected, respectively, to the +B terminal through a resistor and an inductance member the purpose of which is to produce high frequency compensation. The anode of the tube 15 is connected to the +B terminal through the primary winding 'l5 of the transformer Tl., the secondary winding 18 of which is connected in a series circuit including the horizontal sweep coil and two equal, series-connected resistors 'I9 and 8D, the common terminal 8| of which is connected to ground and the external terminals of which are connected to the two terminals s2 and 83 .of the squarer circuit 58. The terminal 83 is also connected through the negative feedback connection 34 and the parallel-connected .resistor 85 and condenser 86 to the cathode of the tube 13. This feedback connection tends to make the sweep current wave in the sweep coil match the shape of the sweep voltage wave input to the amplifier 55,

Each squarer circuit, as shown in Fig. 3, comprises a two-tube arrangement, each tube operating on the curved portion of its characteristic so that the output wave is substantially the Square of .the input wave. The inputs of the two tubes are in push-pull and the outputs are in parallel. Connected across the terminals G5 and 58 is the double potentiometer represented by the resistors 81 and y83, the common terminal 89 of which is connected to ground. Similarly, connected across the terminals 82 and 83 is a double potentiometer comprising resistors Se and 9|, the common terminal 92 of which is also connected to ground The inner variable terminals of the resistors B1 and 3.8, respectively, are connected to the control elements of tubes 93 and 94 while the inner variable terminals of the resistors te and '9| are connected, respectively, .to the control elements of the tubes '95 and 9S. The cathodes .of all the tubes 93, 94, 95 and Ti are connected together and the common terminal is connected through a resistor B1 to ground. The common terminal of the cathodes is also connected through resistor S8 to the -i-B terminal. biased until they are near the cut-oli potential oi the tubes, since the resistors 98 and 97 comprise a potentiometer connected between +B terminal and ground. All of the anodes of the tubes 93, 94, 95 and s6 are connected together and their common terminal is connected through resistor 99 to the +B terminal. The common terminal oi the anodes is also connected through appropriate coupling members to the control grid of the tube ist in the correction amplier 69.. The characteristic curve of each. of the pairs of tubes 93, 9d and 35, S6 is a parabola (due to the fact that the two characteristic curves or each pair are added in reverse manner since the inputs are of opposite polarity and the outputs are added directly). Hence the unbalanced output of each ypair of tubes is :the square of .the balanced input to that pair of tubes, Since the output currents of the squarer-s e] and et are passed through the :same load resistor 99, the

This causes all the cathodes to be tube, acting as a cathode follower,

output voltage applied to the coupling .condenser is proportional to the sum .of the squares of the input voltages from the horizontal and vertical sweep ampliiiers 54 and 55. By means oi the movable terminals for the resistors B7, 88, 9E) and 9|, balanced gain control can be obtained.

The correction .amplifier 69 comprises two .tubes |59 and |2 to the former one of which the combined output of the squarer circuits 61 and 68 is applied. The anode of the tube |08 is coupled to the control element of the tube |62, and also through coupling condenser |83 to a terminal |04 of the vacuum tube voltage divider circuit i3 shown in Fig. 4 and which will be described in detail below. The anode of the tube |02 is connected to a terminal |05 of the voltage divider circuit 43 through a coupling condenser |85 The two outputs of the correction amplifier 69 are thus applied to the voltage divider circuit 43, so that a variable component can be applied to the rings 23 to 31, inclusive, in addition to the direct component also applied thereto from the voltage divider circuit. Because the terminals IM and |95 of the correction amplier S9 are connected to different stages thereof, the outputs are of opposite polarity (but they are not balanced due to the fact that there is gain in the second stage). The voltage divider circuit shown in Fig. 4 has a negative part (comprising the two tubes on the left and the circuits associated therewith, and a positive part (the rest .of the circuit). The pick-up nger 39 and the ring 25 nearest it are connected to ground which is the potential of the terminal common to the left and right parts of the circuit 13.

A high voltage, direct current power supply |01 having its positive terminal grounded is associated with the positive part of the vacuum tube potentiometer circuit shown in Fig. 4. Between ground and the negative terminal of the power supply |07 are connected in series a resistor IBB and the anode-cathode circuits of a number of vacuum tubes |69 to |29, inclusive. The anode of tube HIS is connected to ground and `each of the tubes I0 to |28, inclusive, has its anode connected to the cathode of the preceding tube. The cathode of tube |62 is connected through resistor |538 to the negative terminal of the power supply ID?. As a result, the anode-cathode paths of the tubes |519 to |23, inclusive, are all in series and if no current is drawn by the grids of these tubes, or from any of the terminals of these tubes shown connected in Fig. 4 to rings 25 to 32|, inclusive, the .same plate current will flow in .all the tubes.

The grid of each of the tubes les to |23, inclusive, is connected to the corresponding `iunction point between a pair of the .resistors 12| to |33, inclusive, which resistors are connected in series between ground and the negative terminal of the high voltage supply. As a result, each grid of the series of tubes |09 to |2E!, inclusive, is placed at a more negative potential than the grid of the preceding tube, and if the resistors |24 to |32, inclusive, have equal resistances, the potential differences between the grids of successive tubesl will all be equal and each will be equal to the potential diference between the grid of tube le!! and ground. Since the cathode potential ci each is slightly more positive than the grid potential of the same tube, the potentials oi the output .terminals which are connected respectively to the cathodes of tubes |69 to i241, inclusive, form a set of potentials decreasing progressively from ground (potential of ring 25) to a large negative potential (potential of ring 3l).

The common terminal of resistors |32 and |33 is connected to terminal |55 of the correction amplifier 39. Each of the resistors |2| to |32, inclusive, is shunted by a capacitor I3@ to |65, respectively. If all the resistors |2| to |32, inclusive, are equal and all the capacitors |313, to M5, inclusive, are equal and large compared to the stray capacity of the grids of the tubes W9 to |25, respectively, then any alternating potential wave applied between terminal IE and ground will appear in equal parts across each resistorcapacitor combination just as the total deflection of a coiled spring appears in part across each turn. The total alternating potential aprpearing on any grid will therefore be proportional to the number of resistor-capacitor combinations between that gridl and ground and therefore also proportional to the direct potential of the grid. The superposed alternating potential variations of the grids are communicated to the output terminals,'each tube acting as a cathode follower. By this means, the rings (25 to 35 inclusive) between the cathode 22 and the pick-up finger 39 have applied thereto direct voltages which are progressively less (more negative) in the direction of the cathode and each ring has applied thereto a proportional alternating component which varies in accordance with the sum of the squares of the horizontal and vertical sweep voltages. The electric field formed by these ring voltages is extended beyond the cathode 22 by means of the ring 31 which is more negative than the cathode 22 and extended in the opposite direction beyond the pick-up nger 39 by means of the rings and 23 to which are applied positive potentials with respect to that of the anode iinger 39 by means of the left portion of the voltage divider circuit 43 shown in Fig. 4. The mesh cap is on or near the end wall of the tube Ml is placed at the same potential as the ring 23 and is used to prevent the electric eld lines from curving up toward the ring 23. This makes possible a more uniform eld distribution within the tube IU.

Referring now to theleft portion of the voltage divider circuit of Fig. 4, the potentiometer comlprising the resistors |45, |41 and |43 is connected between the plus terminal of a direct current supply |54 designated Dissector Voltage Supply #2 (which is usually of lower voltage than the Dissector Voltage Supply #1) and ground. The resistors |41 andY |48 are respectively shunted by condensers |49 and |55. The terminal ma is connected to the common terminal of resistors |45 and |41 and to the grid of a tube |5| the anode-cathode circuit of which is connected in a series circuit with the corresponding circuit of a tube |52 and a resistor |53 across this power supply |54. The common terminal of the resistors |41 and |48 is connected to the control element of the tube |52. The cathodes of the tubes |5| and |52 are respectively connected to the rings 23 and 24.

In the arrangement of Fig. 4, it will be noted that use is made of a cathode follower connection to remove output voltages. This has the eiect of decreasing the impedance as seen by the rings of the dissector tube Ill. Currents drawn by these rings in the dissector therefore have little effect on the potentials of the rings and the stray capacities have little shunting effect on the high frequencies contained in the alternating potential variations.

The operation of the system shown in Fig.4 1 will now be described. An image of the object O is focussed upon the photo-electric cathode 22 by means of the lens system 33 and photoelectrons emitted from this cathode are formed into a beam having a cross-sectional area corresponding to that of the optical image. lThe potentials applied to the rings 23 to 35, inclusive, are sufficiently large to produce saturated photo-emission from the cathode 22. For example, the ring 35 nearest the ring 35 at cathode potential is placed at a potential of about 100 volts positive with respect to the cathode and the following rings have progressively larger potentials applied thereto, the difference between two successive rings being approximately 100 volts.

Since the rings in the dissector are uniformly spaced along the axis of the tube, a substantially uniform axial electric eld is produced Vif the potential difference between adjacent rings is the saine for all rings, and provided the cathode 22 and mesh cap 44 are respectively placed at the potentials of the rings having the same axial position. The potentiometer circuit oi Fig. 4 and the construction of the tube Il) vare such as to permit these conditions to be met. As the potentials of the ungrounded rings (and cathode and mesh cap) are varied in accordance with the input to the correction amplifier, all these potentials vary proportionately, so that while the electric eld is made to vary in intensity with time, the eld is always substantially uniform throughout the volume of the dissector.

rIhe uniform axial eld set up by the focussing coil I3 in cooperation' with the substantially uniform electric eld produced by the potentials applied to the conducting rings 23 to 31, inclusive, and the cathode 22 and mesh cap 44, bring the photo-electrons to a focus in the plane of the aperture in the pick-up member 39,A thus forming an electron image of the object in this plane. The horizontal and vertical sweep coils and I2 produce magnetic elds uniform in intensity throughout the dissector volume and in directions transverse to the axis of the dissector. Each eld, which is continuously varying with time due to the saw-toothed current applied to the coils, sweeps the entire electron image across the aperture of the pick-up member 39, thereby allowing successive elements in the electron image to pass through the aperture. The output of the pick-up member 39 is amplified and prepared for transmission over the antenna 48 in a manner well known to the workers in the television art. If a motion picture film is the object O, one of the sweep coils is not used, one direction of the scanning being supplied by the motion of the lm. Correction for distortion which would otherwise be present is obtained by applying the saw-toothed variations produced in the vertical and horizontal sweep amplifiers 54 and 55 to the squarer circuits 6l and 58, the outputs ofwhich are added and applied to the amplier 59 and the voltage divider 43 whereby the alternating components of the potentials applied to the rings 23 to 3l, inclusive, arecaused to vary in accordance with the sum o f the squares of the two deflecting fields, or, in other words, in accordance with variations in Since Hz is substantially a constant, it will be seen that the electric eld is modulated in a manner indicated by mathematical expression (10) above, or, in other words, correction is ob:

arancio tained ini accordance with the first method of eliminating geometric l distortion.

in thearrangement of` Fig. 1.,y they balanced resistors 62 and 63 (or resistors 19` andA 86) are replaced by a single. resistor |6|, producing an unbalanced voltage output, a phase-inverting circu-itbetweenL this resistor andthe input circuit of the squarer 61 (which is balanced) is necessary. While any suitable phase-inverter can be used, the circuit shown in Fig. is. a sim-ple: and convenient way of performing` this function and it has the advantages that (l) the direct components of the signals are transmitted by the phase-inverting circuit and (2) that both sides of the balanced output producedv are near ground potential. The elements within the box |62 in. Fig. V5` can be usedfto replace'those. in the bex 16S, Fie. 1- A In the arrangement ci Fig. 5, unbalanced in put signals areA applied. between the terminal'f |63 and` ground. Thisterrninalv is connected to one end ofapotentiometer comprising the resistances |66,` |65 and |66, the other end ofV the potentiometer being connected to the output ternfiinal |61 connected to thev squarer circuit 61. rEhe other outputl termi-nal |68 is connected directly tothe input terminal |63.. An inner variable tap |69 of the resistor |65 is connected. to the control grid of the tube |16. The anode of this'Vtu-be is connected through a resistor to the +B terminal of a suitable source 1| of direct potential of, for example, 300 volts. The cathode is connected through a resistor |12 to ground while the anode is connected through a coupling condenser |13 shunted by a resistor |14 which permits the passage ofthe direct components of the signal to the control element of the tube |15. rIfhe cathode of this tube is connected through the` resistor |16 to the negativeV terminal of a suitable source of. potential |11 of about llcvolts, the positive terminal of which is connected to ground. The anode of tube |15 is connected. directly to the positive terminal of thesource |1|. The cathode of'` the tube |15 is connected directly to the terminal |61'.

rilue-operation of the circuit shown in Fig. 5 will now be described. Assume a small direct positive potential is applied between the input terminal |63 and ground. By means of the tap |69 on the resistor |65, a positive potential is applied to the input of the tube which causes a negative potential to be applied to the grid of the tube |15. Because of the cathode follower output connection, the terminal |61 also assumes a negative potential. By properly adjusting the position of the tap |69 the output at the terminal |61 can be placed as far below ground as the input terminal |63 (and hence the output terminal |68) is above ground.

Fig. 6 shows a circuit diagram making use of the second method of correction, that is, both focussing and deecting fields are modulated by a factor proportional to the sum of the squares of the two deilecting fields. The manner in which this produces correction is brought out by Equation 1l above. In the arrangement of Fig. 6, those elements which have similar characteristics to corresponding elements in Fig. 1 have been given the same reference characters as those of Fig. 1. The outputs of the vertical sweep generator 52 and the horizontal sweep generator 53 are applied respectively to modulators |86 and |8|, the outputs of which are applied to the vertical and horizontal sweep amplifiers 54 and 55 shown in Figs. 3 and 4. As in Fig. 1, the cutputs of these amplifiers are applied, respectively, to the vertical and horizontal sweep coils I2 and l! and voltages proportional to the sweep currents are applied respectively to the input circuits of squarers 61 and 66, the outputs of which are combined and applied to an amplifier |82. These outputs are also combined and applied to an` amplifier |63 which has its output circuit connected to the focussing coil f3. The output of the amplifier E52 is applied by means of the terminal |64 to the modulators 666 and |`8|. The amplifiers |52 and |63 are of any Suitable type or types. A suitable modulator circuit for the member i60 or the member |'8`| is shown in Fig. 7L y Considering now the modulator shown in Fig. 7 the output of the sweep generator 52 is applied to the input'winding |85 of a transformer |66, the secondary winding |81 of which has' its outside terminals connected respectively to the control elements of the tubes |85 and |89 and its mid-terminal connected through a resistor to a point at +B potential. The anodes of the tubes |68 and |69 are connected respectively to the outer terminals of the primary winding |91 of a transformer |92, the secondary winding 93" of which is connected to the sweep amplifier 511 shownin Fig. l. The mid-point of the transformer winding` |,5E` is connected directly to the +B terminal. The cathodes of the tubes |86 and |83. are connectedtogether and the common terminal is connected to ground through resistor |94; This common terminal is also connected to the output. terminal ofthe amplifier |82. The signals applied to the cathodes of the tubes |86 and i89 modulate. the input signals applied to the control elements of these tubes so that the vertical sweep amplier 511,. and hence the vertical sweep coil i2, has applied thereto signals which produce the desired' deflection and also correct for the` distortion in the manner indicated by Equation 11 above. Similarly, the modulator |8| receives signals from the horizontal sweep generator 55:2v and from the output terminal of the amplifier |32 and modulates the signals applied to the horizontal sweep coils with the correcting signals. These correcting signals are also applied through the amplifier |83 directly to the focussing coil I3 so that a correcting component proportional to the sum of the squares of the deiiecting voltages is applied to each of the deflecting coils I and i2 and to the focussing coil |3 to produce correction for geometric distortion in accordance with the second method (in the manner indicated in Equation 11). If desired, an auxiliary focussing coil, preferably having fewer turns than the main focus coil I3, can be connected to the amplifier |83 (instead of the main focus coil |3 shown in the drawing). The rings 25 to 31, respectively, of tube |0 have applied thereto the same direct components as in the arrangement of Fig. l but these components are not modulated with a correcting signal as in Fig. l.

Various other modications can be made in the various embodiments of the invention described above without departing from the spirit thereof, the scope of which is indicated in the claims.

What is claimed is:

l. In combination, a cathode ray television transmitter tube of the type containing a cathode and an anode and wherein a beam of relatively large cross-sectional area compared with the size of an elemental area of an object to be televised is formed, means for producing an electric assumo 19 field between said cathode and anode for accelerating said beam within said tube, means for forming a beam-focusing magnetic eld, magnetic eld-forming means for delecting the beam in at least one coordinate direction, and means for greatly minimizing the image distortion normally produced in said tube comprising means for varying the strength of at least one of said iields by an amount proportional to the square of the total deecting eld.

2.4 In combination, a cathode ray television transmitter tube or the type containing a cath- Y ode and an anode and wherein a beam of relatively large cross-sectional area compared with the size of an elemental area of an object toA be televised is formed, means for producing an electric eld between said cathode and anode for accelerating said beam within said tube, means for forming a beam focusing magnetic field, magnetic field-forming means for deecting the beam in at least one coordinate direction, and means for greatly minimizing the image distortion normally produced in said tube comprising means for varying the strength of the electric field by an amount proportional to the square of the total deecting field.

3. In combination, a cathode ray television transmitter tube of the type containing a cathode and an anode and wherein a beam of relatively large cross-sectional area compared with the slze of an elemental area of an object to be televised is formed, means for producing an electric eld between said cathode and anode for accelerating said beam within said tube, means for forming a beam-focusing magnetic eld, magnetic eldforming means for deflecting the beam in two coordinate directions at right angles to one another, and means for greatly minimizing the image distortions normally produced in said tube comprising means for Varying the strength of the electric eld by an amount proportional to the sum of the squares ofthe two defiecting fields.

-4. In combination, a cathode ray television transmitter tube of the type containing a cathode and an'anode and wherein a beam of relatively large cross-sectional area compared with the size of an elemental area of an object to be 'televised' is formed, means for producing an electric eld between said cathode and anode for accelerating said beam Within said tube, means for forming a beam focusing magnetic eld, magnetic eldforming means for deecting the beam in at least one coordinate direction, and means for greatly minimizing the image distortion normally produced in said tube comprising means for varying the strength of both the focusing and deecting fields in accordance with a function of the total deflecting eld. Y

5. In combination, a cathode ray television transmitter tube of the type containing a cathode 'and an anode and wherein a beam of relatively large cross-sectional area compared with the size of an elemental narea of an object to be televised is formed, means for producing an electric eld between said cathode and anode for accelerating said beam within saidV tube, means for forming a beam-focusing magnetic fieldr magnetic held-forming means for delecting the beam in two coordinate directions at right angles to each other, and means for greatly minimizing the image distortions normally produced in said tube comprising means for varying the strength of both the focusing and deflecting elds by am amount proportional to the sum of the squares of the two deiiecting fields.

BERNARD M. OLIVER.

REFERENCES CITED The following references are of record in the Y ille of this patent: 

